The antenna is the electrical device which inputs, that is receives, radio frequency (RF) electromagnetic (EM) waves radiated through space to the location of the antenna and couples the RF to an associated RF receiver for processing and/or sends, that is radiates, RF produced by an associated RF transmitter. In the simplest physical form an antenna may be formed of one or more lengths of electrical conductors which serve as "radiating elements" or radiators. To this elemental structure may be added additional metal or dielectric elements to support the radiating element and modify its electrical characteristics. The reader may refer to the technical literature for further details of the design and construction of antennas in general, such as Antenna Theory & Design, Stutzman & Thiele, 1981, published by John Wiley & Sons and Antenna Theory & Design, Constantine A. Balanis, published by Harper & Row 1982, all of which are known to those skilled in the art. One of those antennas is the broadband "spiral" antenna, an antenna characterized by a length of conductor that is of a spiral shape having application in present electronic countermeasures systems.
Target seeking missiles and military aircraft employ search radars that transmit RF energy to search for a target. The RF may be of any frequency within a broad range and may be of a polarized form that makes the RF wave more difficult to detect, either right hand or left hand elliptical or circularly polarized. To thwart this electronic threat the target aircraft must have the means to detect and then confuse or deceive the search radar, which are referred to as electronic countermeasures systems or, more simply, ECM systems. To be effective in the present electronic warfare environment an ECM system for military aircraft should be able to instantaneously sense incident RF energy over a broad frequency range, approaching a decade of frequency, 2 GHz to 18 GHz by way of example, to ensure detection of RF emitted by the search radars of hostile aircraft and missiles.
The EM waves used in this application as is known may be polarized and that polarization can vary from linear to circular. Moreover the circular polarized EM waves may have a right or left elliptical or circular polarization. A right circularly polarized antenna structure will not detect a left circularly polarized wave and vice versa. Thus, an electronic countermeasures system equipped with conventional sensor antennas, such as broadband spiral antennas, would be `blind` to some threats.
To attain the capability to meet the aforedescribed electronic threats present day countermeasures systems must be modified to double the number of spiral antennas and associated circuitry to detect both right and left elliptically polarized EM waves. Even if the increased cost were to be disregarded as a factor, space on military aircraft is at a premium or is simply unavailable and additional equipments cannot easily be accommodated. A need exists for an antenna of simple structure capable of detecting both kinds of polarized EM waves to replace the less capable antenna in ECM systems without the addition of space and weight.
U.S. Pat. No. 3,681,772 granted Aug. 1, 1972 to P. G. Ingerson for a "Modulated Arm Width Spiral Antenna" shows that it is known to provide an antenna that detects EM waves in both senses of circular polarization. U.S. Pat. No. 4,243,993 granted Jan. 6, 1981 to B. J. Lamberty et al. for a "Broadband Center Fed Spiral Antenna" shows another antenna for that same purpose. These structures take advantage of a "converted mode" to detect EM waves of both kinds of polarization. The converted mode operation is attained by a series of impedance discontinuities or reflection regions along the antenna arms which selectively reflect the antenna currents. This reflection "converts" the sense of polarization from right to left and conversely. The disadvantage of the foregoing antenna lies in its inability to effect total reflection of the excitation currents. Residual currents cause radiation of the opposite sense of polarization, increasing the axial ratio and degrading the radiation pattern and antenna gain.
Others have previously discovered that an antenna structure could be made compact in size and essentially planar in form. This compact planar design permitted efficient use on aircraft and also permitted assembly by "metal on insulator" plating techniques familiar to those skilled in the printed circuit board art. Thus for example the spiral antenna referred to earlier has been duplicated in planar form as is depicted in the publication no. "Antenna 587-1" published by the Military Electronics & Avionics Division of TRW, Inc., the assignee of the present application.
The log periodic antenna, another kind of antenna structure familiar to many in one form as a TV antenna, which is of linear polarization, has also been produced in compact planar form. An example of the structure of such a planar log periodic antenna is presented in the advertisement of AEL appearing in their catalog #6847.5MR illustrating model APO 1466. As the advertisement states: "The log periodic as well as equiangular spiral antennas can be made in planar form. Such an antenna exhibits excellent frequency independent free space radiation patterns. . . . this linearly polarized antenna can be flush mounted for airborne application, used as parabolic reflector feed, and for general purpose low profile installations."
A characteristic feature of the AEL antenna is that it is cavity backed, that is the planar surface, like a pot lid, is supported in a pot shaped metal container, that defines a high frequency tuned cavity. As those familiar with this antenna recognize the cavity backing serves to prevent RF leakage from the antenna's underside into the aircraft carrying the antenna as well as to synergistically enhance the electrical characteristics of the radiating elements forming the antenna.
The aforedescribed AEL antenna as illustrated contains two electrically conductive metal stems arranged on a common axis that extends through the center of a circular shaped base, the latter of which is formed of dielectric material, with the stems being in opposed end to end relationship about said center. Each of the stems contains transversely extending branches; more specifically there are a plurality of conductive metal branches spaced from one another along the length of and extending transversely from each stem. Each of the branches forms a circular arc, extending almost over a quarter of the circle. Odd numbered ones of those branches extend to the right of the stem and even numbered ones extend to the left of the associated stem. Each of the two conductors serves as a radiating or receptor element of the antenna with the overall characteristic of the antenna being principally determined by the effect of the combination of the two radiating elements.
The physical shape of the linearly polarized antenna is simple and beneficial. Linear polarization is useful in some applications where the polarization is known or where it is desirable for the antenna to discriminate against the cross polarized EM field components.
An object of the present invention is to provide an antenna structure that allows detection of both right and left hand circular polarized RF energy, without resorting to the "converted mode." A further object is to provide an antenna structure that allows easy installation of the transmission lines and which eliminates the need for two 180 degree hybrids found in prior designs, resulting in reduced cost and complexity. An additional object of the invention is to provide a dual polarized antenna structure that eliminates the broadband unbalanced to balanced transitions required in prior dual polarized antenna structures. A still further object of the invention is to provide an improved planar antenna and feed structure that enhances linearly polarized and circularly polarized antennas.